Border gateway protocol (BGP) is a protocol for exchanging routing and reachability information between autonomous systems (ASes). An AS is a set of routers under a single technical administration. An AS typically employs an interior gateway protocol (IGP) to exchange network topology information among routers within the AS. An AS may employ more than one IGP and in some cases more than one instance of a given IGP.
Border gateway protocol link state (BGP-LS) uses BGP as a carrier for network topology information collected by an IGP. BGP-LS allows a BGP speaker to share network topology information collected by the BGP speaker (e.g., network topology information collected using an IGP) with external components (e.g., another BGP speaker located in another AS) using BGP. BGP-LS defines a link-state network layer reachability information (NLRI) encoding format that is used to share network topology information with external components. Each link-state NLRI describes either a node, a link, or a prefix. Network topology information is aggregated based on AS number, link state identifier (LS-ID), protocol, instance identifier, and NLRI type.
BGP route refresh capability allows a BGP speaker to dynamically request a re-advertisement of network topology information from a BGP peer. A BGP ROUTE-REFRESH message is encoded as:

Performing a route refresh using the existing ROUTE-REFRESH message triggers a refresh on an entire address family identifier (AFI) and subsequent address family identifier (SAFI) specified in the message. This causes a refresh of all network topology information for a given AFI and SAFI, which may result in unnecessary churn and communication overhead.